Telephone connections in which one or more hands-free devices, such as speakerphones, are connected over conventional telephone networks are faced with a classic problem of controlling acoustic echo. Echo is often introduced at the hands-free devices by virtue of the close proximity of a microphone and a speaker and results in the received acoustic signal to be transmitted on the return path. This acoustic "feedback", however, is delayed by a finite amount of time, usually on the order of milliseconds. This acoustic signal is received and heard by the speaking party as an objectionable echo of the speaking party's voice.
Voice-switching, also known as switched-loss, is a commonly used technique to reduce echo. In this technique, the echo control unit inserts loss into either or both paths of the circuit connection, depending on which party is determined to be speaking at that particular time. In a speakerphone, the loss is inserted into the transmit path when the far-end user is talking (the speakerphone receive signal is declared as active). When the near-end user is talking, the loss is placed in the receive path and if neither is talking the loss may be in either path or split between them. In all cases, the loss contributes directly to control of the echo signal. When both parties are talking--such as the case where one user attempts to interrupt the other--voice-switching results in the clipping or attenuation of one or both user's speech signal(s). In effect, the connection is only half-duplex. Some hands-free devices offer full-duplex connections by using acoustic echo cancellers within the echo control unit with algorithms to model the delay path and subtract out the echo signals such that the signals transmitted from the devices do not contain a significant amount of echo. Such algorithms typically provide up to 25-35 dB of echo control with little or no supplementary voice-switching or loss due to non-linear processing which might otherwise render the call half-duplex. This amount of echo control is adequate in most conventional wireline (vs. wireless) telephone connections.
As an alternative to conventional circuit-switched networks, LAN/WAN networks and other IP networks have been employed increasingly to communicate voice and to provide for telephone connections. Accordingly, a connection over a LAN/WAN network may include one or more hands-free devices, such as speakerphones, which can potentially introduce objectionable echo into the LAN/WAN telephone connections. The round-trip delay introduced in these networks varies, and when significant, places more severe requirements for control of such echo to prevent it from being objectionable to users. In fact, for aroundtrip delays up to a few 100s of milliseconds, the requirement for echo control rises commensurate with the amount of delay in the call. In cases of high delay, greater than 100 msec for example, the required amount of echo control will be beyond the capabilities of today's acoustic echo cancellers--without the addition of adequate voice-switching or non-linear processing loss which will render such calls half-duplex. This supplementary (voice-)switched or non-linear processing loss will be denoted as signal-dependent loss.
The strategy in current high-delay voice calls, such as those made over digital wireless connections or voice-over-IP, is to add the necessary loss, via voice-switching or some non-linear processing, to achieve the necessary echo control for the worst-case expected round-trip delay. In the process, full-duplex operation is sacrificed on all calls. In LAN/WAN networks, however, the amount of delay in a call varies depending on network conditions and types. Thus the echo control strategy designed for worst-case expected delay tends to penalize call quality ("full-duplex-ness") on those calls with low delay.